1. Field of the Invention.
This invention relates to stepper motors which are controlled by means of electronic commutation, and more particularly, to the use of stepper motors in incremental motion in a closed loop mode. Still more particularly, this invention relates to the use of stepper motors to provide paper drive in high-speed printer systems.
In order to increase the throughput of a printer, either printer time, housekeeping time, or paper feed time must be reduced. In order to provide the required print quality, it is desirable to maintain print time at a predetermined level. Therefore, an increase in throughput without a resulting decrease in print quality is best obtained by reducing the paper feed time, i.e., the time to move from one print line to the next. The motion must be well controlled despite the fact that the load is subjected to a wide range of disturbing torques such as friction and paper weight.
2. Description of the Prior Art.
Although most stepper motors are used in an open loop mode, some designs have incorporated such motors into closed loop systems. One such system is disclosed in U.S. Pat. No. 4,136,308, issued to Kenyon King on Jan. 23, 1979. This patent discloses a method by which a dc power source is switched successively in four stator windings of a four-phase bifilar stepper motor. Switching time in relation to shaft position is controlled by sensing the back electromotive force (EMF) in the stator windings to provide signals indicating the position of the rotor of the motor. The back EMF sensing circuit is a transformer with a winding in series with two windings of the stator and another winding in series with the other two windings of the stator. A pulse is produced each time the voltage across the two transformer windings are equal. These pulses are used to generate clock signals for sequencing the switching (commutation) of the stator windings. The signals provided by the sensing circuit correspond to an angular position of the motor at which switching must occur to obtain maximum torque (either acceleration or braking) from the motor.
The above-described system has two main disadvantages associated with it. The first is the need for a special transformer to generate the position indicating signals. Second, the method of control of the motor is unreliable when used in incremental motion in the presence of disturbing torques such as friction. Over and above these drawbacks, the control system for this motor is such that the motor is initially operated in an open loop mode until sufficient speed is obtained to enable the transformer to generate a useable sensing signal. During this mode of operation the system is vulnerable to disturbing torques.
Other stepper motor control systems are disclosed in U.S. Pat. No. 4,126,821 to Cannon, U.S. Pat. No. 4,234,838 to Langely, et al., and U.S. Pat. No. 4,262,240 to Arai. None of these systems is well suited for providing incremental motion in a paper feed system.